1. Technical Field
The present invention relates to electric motors having a very limited rotation, for example in the range of about ±12° (degrees). Such motors are sometimes called galvanometers or rotary actuators.
2. Description of the Background Art
The dominating use of rotary actuators is in moving magnetic heads to a requested track in a hard disc drive. Most of such actuators use voice coil motors. Voice coil motors are, for example, used in the disc drive systems described in U.S. Pat. Nos. 5,329,267, 5,448,437, 5,459,362, 5,537,270 5,608,592, 5,621,591, 5,654,848 5,675,455, 5,694,272, 5,698,911, 5,883,761 and 5,914,836.
However, rotary actuators based on other principles than a voice coil have also been disclosed. The published Japanese patent application 61-154471 for Matsushita, inventor Yukihiro Ashizaki, discloses a disc head rotary actuator having six wound stator poles arranged in two sets of three poles. The three poles in a pole set are each driven by one of three currents 120° out of phase with each other. The stator seems to cover a mechanical sector of about 45°. The rotor seems to have several permanent magnet poles. The published Japanese patent application 61-124254 for Matsushita, inventor Hiroaki Taame, discloses a disc head rotary actuator having three wound rotor poles connected to different phases. The stator seems to cover a mechanical sector of about 45° and seems to have several permanent magnet poles.
U.S. Pat. No. 4,968,909 for Rand H. Hulsing, II discloses an actuator having two wound stator pole pairs in the centre and a soft magnetic rotor that can be attracted to either of the stator pole pairs. The motor is fully symmetric around the rotor shaft. There are three separate windings and no permanent magnets. In U.S. Pat. No. 5,270,594 a similar basic design is disclosed.
U.S. Pat. No. 5,025,201 for Alexander Berger discloses a resolver having a 10° range and a stator covering a sector of some 90°. There are no permanent magnets. As is normal for a resolver, there are two static and one moving phase winding, and the magnetic coupling between the moving and the static windings depends on the position of the rotor.
U.S. Pat. No. 5,038,062 for Manabu Shiraki discloses what can be described as an inverted voice coil arrangement. In the conventional voice coil arrangement a rotor having one or two flat coils located in the same plane moves in an air gap with an axial magnet field generated by two stator magnet sets. The magnets face a smooth back iron over a radial air gap that is approximately of the same length in the direction of flux as are the permanent magnets and the single or dual rotor coil(s) move in this air gap. The magnet sets are parallel to the rotor movement plane. Instead, in U.S. Pat. No. 5,038,062 a rotor with one magnet moves between two stator flat coil pairs parallel to the rotor movement plane. The rotor magnet causes an axial flux in the air gap, and the flat coils are inserted in this air gap. The four coils are connected to the same phase.
The published Japanese patent application 2-074143 for Fujitsu, inventor Hiroshi Maeta, discloses a design of a rotary actuator that also can be described as an inverted voice coil arrangement. The magnets are moving also in this design but the air gap flux is radial. Basically there are four moving magnets with equal sectors. Two of these are mounted in contact with each other and can be replaced by a single magnet covering a sector that is twice that of the two other magnets. Like the conventional hard disc voice coil motors, the coils are inserted in the air gap between the moving magnets and a smooth back iron. The basically two coils are static, and all coils are connected to the same phase. The published European patent application 0 127 058 for BASF, inventors Klaus Manzke et al., discloses a similar arrangement having two coils in the radial air gap between four magnets and a smooth back iron. The two coils are connected to the same phase.
The published German patent application 19 816 201 for Seiko Instruments, inventors Takashi Ishida et al., discloses a rotary actuator having a rotor carrying permanent magnets creating a flux in the radial direction. Around the rotor at least two ironless coils are arranged. The coils are either wound around a common non-magnetic structure (for example made of a thermoplastic) or wound one by one and inserted in slots in a nonmagnetic stator coil fixture. All coils are connected to the same phase. The stator and rotor are symmetric around the shaft and covers a 360° sector.
U.S. Pat. No. 5,557,152 for Raymond G. Gauthier discloses a rotary actuator having one or two coils in the stator. Both coils are connected to the same phase. The rotor carries one or two permanent magnets and the flux in the air gap is axial. The rotor also carries the back iron required by these magnets. Unlike the other single phase designs described above, there are stator poles made of a magnetically highly permeable material, and the stator coils are placed around these stator poles. The design shown uses a very large air gap (about 2.5 mm) to reduce the otherwise enormous cogging torque that is inherent in the design.